Abstract
Well engineered piezoelectric actuators characteristically show a high energy density, but there is still potential for improvement in transferring the energy from the piezoelectric element to the moving component (e.g. rotor). This transfer is basically determined by the friction between stator structures and the rotor. In prior publications a cilia based piezoelectric actuator was proposed, which uses a microscopic friction layer in this transfer procedure.
The cilia based piezoelectric actuator uses a layer of carbon fibres with a defined inclination as a friction layer for the anisotropic energy transfer from the stator to the rotor. The stator structure is formed by a piezoelectric ceramic element (piezo stack), the stator structure, and the fibre layer, which is attached to the surface of the stator structure. As the piezo stack is activated, the stator structure amplifies its stroke and the fibres, which now are perpendicularly pressed against the contact surface of the rotor, set the rotor in motion tangentially to the fiber?s motion axis. As this happens, the fibers are elastically bent. Through the anisotropic friction resulting from this contact, the energy is transferred from the stator structure to the rotor. Thus, the rotor is set in motion.
The interaction between the layer of fibers and the contact surface is a distinctive mark of the cilia based piezoelectric actuator. The reliability of the actuator depends strongly on the friction layer. The layer has to show a high mechanical resistance against frequent load stresses and a good matching with the contact surface for a high friction coefficient. To fulfil these requirements carbon fibres were chosen. Carbon fibres have very low masses and excellent mechanical characteristics. The high tensile durability and short length allows achieving very high resonance frequencies. In order to exploit these properties, the fibres have to be aligned in a straight way with a defined inter-fibre space. Doing so, additional friction between fibers is avoided and their elastic deflection is assured. The matching between the fiber layer and the contact surface can be experimentally investigated by measuring the transmitted energy from the fibers to different contact surfaces.
The fiber layer plays a very important role in the functionality, energetic efficiency and lifetime of the proposed actuator. Therefore, a reliable production of this layer has to be investigated.
In this paper, a method for the arrangement of carbon fibers into a 2D array is presented. Nickel coated fibers are used as source material, which are mechanically aligned in a parallel way and embedded in a polymer matrix. At this point, the fibers are mechanically treated with conventional cutting methods in order to achieve fibers with a defined inclination and length. After chemical adhesion between this matrix and a stainless stell substrate an etching procedure is executed to eliminate the nickel coating and the polymer matrix without harming the fibers or the substrate. With these steps, a pillar 2D array of carbon fibers with a defined space between the fibers is achieved. This space depends directly from the fiber coating thickness, which in this case is about 0,5 \mum for a fiber diameter of 7 \mum. Thinner nickel coatings on carbon fibers are commercially avaiable. To increase the spaces between fibers, carbon fibers with a thicker nickel coating can be arranged by manufacturers.
The length of the layers that were manufactured in this work, the fiber distribution and the inter-fiber space were analysed with SEM microscopy.
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